Sputtering is a method of depositing a film on a substrate by accelerating ions typically by means of glow discharge so as to hit a sputtering target so that the target ejects a film material due to the kinetic energy of the ions. It is widely used as a tool for fabricating structures such as thin film devices in the areas of semiconductors, liquid crystals, solar cells, and so on. Among others, a target using a magnesium oxide sintered body (hereinafter, also referred to as a “magnesium oxide target,” a “magnesia target,” and an “MgO target”) is used as a film material for a tunnel barrier layer of a tunnel magneto-resistive element (TMR element) used for magnetic heads of hard disks, high performance non-volatile memories, and the like, and also as a source of film materials in the applications of forming a protective film or an insulating film in a plasma display panel (PDP).
A TMR element, which is an element for detecting changes in external magnetic force by exploiting the tunnel magneto-resistance effect, has an extremely thin insulator layer (tunnel barrier layer) of not more than a few nanometers sandwiched by electric conductors (electrodes). It is generally understood that a TMR element having a higher electrical resistance change rate (an MR ratio) can detect magnetic force changes at a higher sensitivity, thus exhibiting higher performance, and as a tunnel barrier layer material, magnesium oxide is expected to be particularly promising.
Since a magnesium oxide target is usually an insulating material, a high-frequency sputtering apparatus is used for forming a magnesium oxide sputtered film. In this apparatus, a magnesium oxide target is joined to an electrode, a substrate is disposed at a counter electrode of the electrode, and a sputtering discharge is carried out in an atmosphere such as argon under a reduced pressure, thereby depositing a film on the substrate.
Patent Document 1 discloses an invention relating to “a magnesium oxide sintered body sputtering target, which has a sintered density close to the theoretical density, and therefore releases a reduced amount of gas, and which has a crystal orientation in the (111) plane orientation, and thereby enhances secondary electron emission during sputtering.” It is suggested that the magnesium oxide sintered body sputtering target of Patent Document 1 is made up of a sintered body in which a large proportion of crystals have the (111) plane orientation in the plane to which a uniaxial pressure is applied, and that the secondary electron emission during sputtering is enhanced and thereby the sputtering efficiency is improved. Further, in embodiments thereof, it is described that the average grain size is about 10 μm in any case.
Patent Document 2 discloses that, by preparing a magnesium-oxide sputtering target having a purity of not less than 99.50% and less than 99.99%, and a relative density of 97.5% to 99.5%, the vapor deposition rate of the magnesium-oxide sputtering target may be improved. Also reported in a comparative example thereof is a case of a 99.99% sintered body which is fired at 1650° C. in an atmospheric furnace.
Patent Document 3 proposes a method of efficient aqueous granulation of magnesium oxide powder, in which cubic-shaped magnesium oxide having a purity of 99.985% and a specific surface area of 7.5 m2/g, which is obtained by a gas-phase oxidation reaction process, is granulated and formed by using polyethylene glycol and ammonium polycarboxylate, and fired at 1650° C. in an electric furnace to obtain a sintered body of a relative density of 96.1%.